feat: 添加时区设置功能，默认时区改为 CST-8

- 新增 set_timezone LLM 工具，支持通过对话设置时区
- 新增 set_timezone / timezone_show CLI 命令
- 默认时区从 PST 改为 CST-8（中国标准时间 UTC+8）
- 支持 POSIX 格式和 18 个城市名映射（Asia/Shanghai 等）
- 时区通过 NVS 持久化存储（system_config namespace）
- config_show 中显示当前时区配置
- 更新 changelog.md 和 taolun.md 文档

main/onboard/onboard_html.h

@@ -56,10 +56,14 @@ static const char ONBOARD_HTML[] =
 "<label>Model</label>"
 "<input id='model' placeholder='claude-opus-4-5' value='claude-opus-4-5'>"
 "<label>Provider</label>"
-"<select id='provider'>"
+"<select id='provider' onchange='onProviderChange()'>"
 "<option value='anthropic'>Anthropic</option>"
 "<option value='openai'>OpenAI</option>"
+"<option value='siliconflow'>SiliconFlow (硅基流动)</option>"
+"<option value='volcengine'>Volcengine (火山引擎)</option>"
 "</select>"
+"<label>Base URL</label>"
+"<input id='base_url' placeholder='https://api.example.com/v1'>"
 "</div></div>"
 
 /* Telegram section */
@@ -133,8 +137,15 @@ static const char ONBOARD_HTML[] =
 "btn.textContent='Scan WiFi Networks';btn.disabled=false;"
 "}).catch(()=>{btn.textContent='Scan WiFi Networks';btn.disabled=false})}"
 
+"function onProviderChange(){"
+"var p=document.getElementById('provider').value;"
+"var u=document.getElementById('base_url');"
+"if(p==='siliconflow'){u.value='https://api.siliconflow.cn/v1'}"
+"else if(p==='volcengine'){u.value='https://ark.cn-beijing.volces.com/api/v3'}"
+"else{u.value=''}}"
+
 "function save(){"
-"var fields=['ssid','password','api_key','model','provider','tg_token',"
+"var fields=['ssid','password','api_key','model','provider','base_url','tg_token',"
 "'feishu_app_id','feishu_app_secret','proxy_host','proxy_port','proxy_type','search_key','tavily_key'];"
 "var data={};"
 "fields.forEach(f=>{data[f]=document.getElementById(f).value.trim()});"